1. Field
The present disclosure relates to a nanogap structure having an ultrasmall void between metal cores and a molecular sensing apparatus and method using the same, and a method for preparing the nanogap structure by selective etching.
2. Description of the Related Art
A nanogap (nanovoid) formed between two or more metallic nanostructures provides a space capable of localizing external optical energy and enables plasmonic binding between metallic nanoparticles to generate very high electromagnetic signal amplification. A metallic nanogap structure may be used widely in various fields, such as photovoltaics, photocatalysis, metamaterials, surface-enhanced spectroscopy, molecular sensing, or the like.
Typically, the methods for fabricating a metallic nanogap may be classified into lithography-based top-down approaches and bottom-up approaches.
The lithography-based top-down approaches may include fabricating a metallic nanogap through a sophisticated lithography process. For example, energy beam lithography or photolithography is used to irradiate a substrate with micro-/nano-scaled beams, thereby forming a metallic electrode pattern, which can be broken down through mechanical control or electromigration to realize nanometer-scaled gaps. In addition, a metallic layer may be deposited on a lithographically patterned template and the template is removed through a lift-off process to form gaps which have, for example, a sub-10 nm gap size.
Additionally, a nano-scaled thin alumina sacrificial layer may be deposited between two metallic layers through an atomic layer deposition process and the exposed unnecessary alumina may be etched chemically through an ion beam milling and a chemical etching to form nanogaps having a size corresponding to a thickness of the atomic layer deposited thin film.
Meanwhile, as alternative approaches to the above-mentioned top-down approaches, some methods for forming nanogap structures through bottom-up approaches have been suggested.
In theses bottom-up approaches, generally linker molecules, such as DNA, block copolymers or ligand molecules may be introduced between metallic nanoparticles as gap-directing molecules to form nanogaps having a size of several nanometers.
Further, as another alternative approach, nanogaps may be formed through a metallic nanoparticle aggregation phenomenon. Such bottom-up approaches are advantageous in that the size, density and interparticle distance of nanostructures can be controlled as compared to the top-down approaches.
However, according to the studies conducted by the present inventors, most lithography-based top-down approaches require the use of the newest micro-/nano-technology or expensive equipments, and thus are not suitable for forming large-area nanogap structures. Moreover, it is required to develop a process for reducing the cost needed for forming large-area nanogap structures.
Meanwhile, the bottom-up approaches should overcome the problems that they require a delicate control over chemical reaction conditions, have low reproducibility (in case of the metallic nanoparticle aggregation method: H. Liu, Z. Yang, L. Meng, Y. Sun, J. Wang, L. Yang, J. Liu, Z. Tian, J. Am. Chem. Soc. 2014, 136, 5332-5341), or are limited in diffusion of molecules to be sensed due to the linker molecules or gap-directing molecules present between metallic nanostructures.